Here, once more, the microscope came to Pasteur's assistance. If, at the period of the rearing of the silkworms, when the mean temperature is always rather high, some mulberry leaves are crushed in a mortar and mixed with a little water, the liquid being left to itself, in twenty-four hours it will be found filled with microscopic organisms; some motionless, resembling little rods or spores joined end to end, like strings of beads, others more or less active, flexible, endowed with a sinuous movement like that of the vibrios found in nearly all organic infusions in process of decomposition. Whence come these microscopic organisms? The facts relating to spontaneous generation indicate that the germs of these organisms were on the surface of the pounded leaf, spread in the form of dust over the instruments used to triturate the leaf, possibly on the mortar, the pestle, or in the water added to the pounded leaves. It is a curious fact, that if the intestinal canal of a worm in full process of digestion be opened, the pounded leaf which fills it from one end to the other will not show microscopic organisms of any kind, but only cells of parenchyma, green granules of the chlorophyl of the leaf, and remains of the air-vessels of the plant. Through the action of the liquids secreted by the glands which line the integuments of the intestinal canal, the germs of organisms are themselves digested or hindered in their development. The digestive functions of silkworms are so active that everything is carried away, destroyed in the same manner as the leaves themselves. But if from any cause the digestion of the worms be impeded or suspended, then the germs introduced with the food into the intestinal canal will give rise to the multiplication of microscopic organisms which are always found in the artificially bruised leaf when mixed with a little water. How numerous are the causes which may check this digestive function of the worm--a function of such importance to a creature which in the space of one month passes from the weight of half a milligramme to that of five, six, seven, or even eight grammes! Pasteur proved that whenever a worm was attacked with _flacherie_, it always had, associated with the food in its intestinal canal, one or other of the microscopic organisms which are invariably to be met with among crushed mulberry leaves. Summing up in a kind of aphorism a series of observations, Pasteur observes: 'Every _ver flat_ is one which digests badly, and, conversely, every worm which digests badly is doomed to perish of _flacherie_, or to furnish a chrysalis and a moth the life of which, through the injury produced by organised ferments, is not normally perfected.'